The present invention relates to an electrostatic ink jet recording method for ejecting an ink composition by utilizing an electrostatic field.
In an electrostatic ink jet recording process, an ink composition (hereinafter, referred to as “ink”) containing color fine particles charged with electricity in a dispersion medium is used, and predetermined voltages are respectively applied to ejection portions of an ink jet head in correspondence to image data, whereby the ink is ejected and controlled by utilizing electrostatic forces to record an image corresponding to the image data on a recording medium.
Known as an example of an electrostatic ink jet recorder is an ink jet recorder disclosed in JP 10-138493 A.
FIG. 4 is a schematic view showing an ink jet head of an electrostatic ink jet recorder disclosed in JP 10-138493 A.
The ink jet head 80 includes a head substrate 82, an ink guide 84, an insulating substrate 86, a control electrode 88, a counter electrode 90, a D.C. bias voltage source 92, and a pulse voltage source 94.
A nozzle (through hole) 96 through which ink is to be ejected is formed so as to extend perfectly through the insulating substrate 86. The head substrate 82 is provided so as to extend in a direction of disposition of the nozzles 96, and ink guides 84 are disposed in positions on the head substrate 82 corresponding to the through holes 96. The ink guide 84 extends perfectly through the nozzle 86 so as for its tip portion 84a to project upwardly and beyond a surface of the insulating substrate 86 on a side of a recording medium P.
The head substrate 82 is disposed at a predetermined distance from the insulating substrate 86. Thus, a passage 98 of an ink Q is defined between the head substrate 82 and the insulating substrate 86.
The ink Q containing fine particles (color fine particles) which are charged at the same polarity as that of a voltage applied to the control electrode 88 is made to circulate through the ink passage 98 from the right-hand side to the left-hand side in the figure, for example, by a circulation mechanism for ink (not shown). Thus, the ink Q is supplied to the corresponding ones of the nozzles 96.
The control electrode 88 is provided in a ring-like shape on the surface of the insulating substrate 86 on the side of the recording medium P so as to surround the periphery of the through hole 96. In addition, the control electrode 88 is connected to the pulse voltage source 94 for generating a pulse voltage in correspondence to image data. The pulse voltage source 94 is grounded through the D.C. bias voltage source 92.
In addition, the recording medium P is held on the insulating layer 92 of the grounded electrode substrate 90 with the recording medium P being charged at a high voltage opposite in polarity to that applied to the control electrode by a charger utilizing the scorotron charger or the like. Consequently, in this system, the recording medium P functions as a counter electrode, and the high voltage applied to the recording medium P becomes a bias voltage.
In such an electrostatic ink jet recording process, in a state where no voltage is applied to the control electrode 88, the Coulomb's attractive forces between the bias voltage applied to the counter electrode 90 and the charged particles (color fine particles) in the ink, the viscosity of the ink (dispersion medium), the surface tension, the resiliencies between the charged particles, the fluid pressure when the ink is supplied, and the like operate in conjunction with one another. Thus, the balance is obtained among these factors in a state where as shown in FIG. 4, the ink Q has a meniscus shape of slightly rising from the nozzle 96.
In addition, the charged particles migrate to move to the meniscus surface due to the Coulomb's attractive forces or the like, i.e., there is provided a state where the ink Q is concentrated on the meniscus surface.
When the voltage is applied to the control electrode 88, the bias voltage is superposed on the drive voltage so that the ink Q is attracted towards a side of the recording medium P (counter electrode) P to form a nearly conical shape, i.e., a so-called Taylor cone.
When a time elapses after application of the voltage to the control electrode, the balance between the Coulomb's attractive forces acting on the charged fine particles and the surface tension of the dispersion medium is broken. As a result, there is formed a slender ink liquid column having a diameter of about several microns to several tens of microns which is called a thread. When a time further elapses, a tip portion of the thread is divided, and as a result, droplets of the ink Q are ejected to fly towards the recording medium P by the electrostatic attraction force.
In the electrostatic ink jet recording process, normally, a modulated pulse voltage is applied to the corresponding ones of the control electrodes 88 to turn ON/OFF the corresponding ones of the control electrodes 88 to modulate and eject ink droplets. Thus, the ink droplets are ejected on demand in correspondence to a recording image.
Hence, the division of the thread is caused at a frequency much higher than the drive frequency for the pulse voltage used to eject the ink droplets. That is, the division of the thread is continuously caused multiple times for a time period required to apply a pulse voltage to the corresponding ones of the control electrodes once. Consequently, one dot on the recording medium P is formed with a plurality of minute droplets which were separately ejected.
In the electrostatic ink jet recording process, this process is utilized. That is, as described in U.S. Pat. No. 4,314,263, a time period required to apply a pulse voltage once (so-called pulse width) is controlled to thereby adjust the quantity of ejected minute droplets (the number of minute droplets) forming one dot. As a result, the uniformity of dot diameters on the recording medium P can be enhanced, and also the promotion of high gradation in the image recording can be realized by carrying out the control or the like for concentration and gradation utilizing the intentional adjustment of the dot diameters.
When the control for the image recording based on such an electrostatic ink jet recording process is carried out, if an area on the recording medium P is an area hot requiring high concentration so much, the control can be suitably carried out.
However, for a high concentration area called a shadow, a pulse voltage with a long pulse width is applied to the corresponding ones of the control electrodes 88 to eject the large quantity of ink onto the recording medium P. For this reason, the controllability for the gradation and the controllability for the dot diameters are reduced in the shadow area. That is, in the image recording based on the above-mentioned electrostatic ink jet recording process, though there are merits in high image quality, high gradation and the like, a problem occurs in that the controllability and the gradation reproducibility in the shadow area are poor.